Experimental and modelling studies on the DNA cleavage by elsamicin A

Abstract

The ability of elsamicin A, an antitumour antibiotic, to cleave DNA in the presence of ferrous iron and reducing agents, has been analysed using experimental and theoretical approaches. Experimentally, the antibiotic causes DNA breakage in the presence of ferrous ions and a reducing agent. The DNA-cleaving activity appears to be partially blocked by the action of superoxide dismutase and catalase. These results indicate that the elsamicin aglycone moiety (chartarin) can be involved in the production of free radicals. We have performed a broad theoretical study based in the quantummechanical framework, which allow us to determine the redox properties of elsamicin that lead to the generation of radical species. Our results clearly show that elsamicin acts as a true catalyst in the production of superoxide radicals. Moreover, it is suggested that the oxidation/reduction mechanism of the aglycone moiety of elsamicin (a lactone), leading to DNA breakage, is different from the mechanism followed by other well-known anti-cancer drugs, whose chromophore is a quinone. [12]. Fig. 1 shows the chemical structure of chartreusin. At first glance, this molecule resembles the anthracycline group of antibiotics: it also contains a chromophore and a sugar moiety. However, different from anthracyclines, the chromophore moiety (chartarin) is not a quinone, but contains a 5,12-dione portion, and bears a peculiar disaccharide bound to position 10 (Fig. 1). Since reduction and auto-oxidation occur in the quinone moiety of anthracyclines [S], it has been proposed that the 5,12-dione portion of chartreusin might be involved in the formation of free radicals [12]. Due to several drawbacks, chartreusin was not selected as an antitumour drug for clinical trials [13], and, until recently, there were no reports of chartreusin derivatives with improved activity [13, 141. Among them, the related antibiotic elsamicin A (Fig. 1) has been isolated from natural sources [13]. Elsamicin A is structurally related to chartreusin, but contains an amino sugar which makes it more water soluble than chartreusin, since at physiological pH this amino group will be positively charged. Elsamicin is about 30 times more potent than Correspondence to J.

Elsamicin A can convert the Z-form of poly[d(G-C)] and poly[d(G-m5C)] back to B-form DNA

“…At identical molar ratios of added drug per base pair the rate of the Z-B conversion is somewhat faster for elsamicin than for either ethidium or daunomycin (Table I and Figure 4). If, as has been recently shown, the size of the antibiotic is a crucial determinant of its efficiency as an inhibitor of the B-to-Z transition (Gilbert et al, 1991), these differences might be a direct consequence of the larger elsamicin site [about four base pairs (Phrraga & Portugal, 1992)l. We have used both kinetic and CD experiments to determine the fraction of right-handed helix as a function of added elsamicin.…”

“…Elsamicin A binds to alternating cytosine plus guanine sequences (Salas & Portugal, 1991;Phrraga & Portugal, 1992), and it is an effective inhibitor of the B-to-Z transition and an effector of the Z-to-B conformational change. If, as is generally reckoned, Z-DNA plays a role in the control of the cell processes, our results suggest a potential mechanism by which the antibiotic might inhibit replicative events and gene transcription, probably leading to its usefulness as an antitumor antibiotic.…”

Electrochemical monitoring of ROS generation by anticancer agents: the case of chartreusin

“…This voltammetry can be directly related with that extensively studied for quinones (Q) which, in aprotic solvents are reduced successively to the corresponding semiquinone anion radical (Qc À ) and the dianion (Q 2À ). 55 On the basis of thermochemical calculations from Párraga et al 37,56 on elsamicin, a chartreusin analogue harboring identical bislactone aglycone (chartarin), the most favored reductive pathway involves one of the oxo groups but with no clear preference for either the C5 or the C12 group. The above one-electron processes would be responsible for peaks C 1 and C 2 , respectively, for which a reasonable representation can be seen in Fig.…”

“…36 Quantum mechanical calculations suggested that only one of the "oxo groups" of the chartarin motif is reduced with no clear preference for either the C5 or the C12 group, the mono-hydroxylated species being subsequently oxidized by molecular oxygen to give the hydroperoxyl (HO 2 c) radical. 37 In order to provide some answers on the whole mode of action of chartreusin (1), the interaction between this compound and dsDNA has been monitored electrochemically in lipophilic and hydrophilic environments using conventional electrochemistry in DMSO solution, and solid-state electrochemistry respectively. Here, potential inputs are used in a twofold function: provide the reducing environment required for the action of 1 on dsDNA 26,35 and generating ROS species.…”

Naphthopyranones – isolation, bioactivity, biosynthesis and synthesis

“…130 Using the combined techniques of biosynthetic engineering and mutasynthesis has given rise to further analogues of potential therapeutic value that are not available readily through synthetic protocols alone. 133 A wealth of pharmacological data has been obtained on chartreusin 86, elsamicins A 88 and B 89 and related compounds 134 allowing a number of members of this family of anti-cancer agents to proceed to phase II clinical trials. 133 A wealth of pharmacological data has been obtained on chartreusin 86, elsamicins A 88 and B 89 and related compounds 134 allowing a number of members of this family of anti-cancer agents to proceed to phase II clinical trials.…”